The present invention relates in general to control of electromechanical motors and in particular to such motors where the motion is based on repetition of small steps.
Small electromechanical motors, such as piezoelectric motors are commonly used in applications with need for extremely fine controlled positioning. Such applications often appear in e.g. portable consumer devices, laboratory equipment etc. There is often also an additional demand for low power consumption, low weight and price. The required motion is typically linear but often a rotating motor combined with a motion-converting mechanism such as a lead-screw is used. The motion range is often in the order of millimeters. There is yet no real miniature motors presented which fulfils all demands above.
Electromechanical motors may be constructed according to a number of different driving mechanisms. One approach makes use of phase shifted operation of different sets of actuators. In such a way, one set at a time can be brought into non-sliding mechanical contact with the object to be moved, while the other set is moved freely to a proper position for gripping the object. The object is thus moved forward by frequently repeating small steps at frequencies below resonance frequency. The motion is thus a non-dynamic or quasi-static motion. One typical mechanism for non-dynamic motion is the xe2x80x9cinchwormxe2x80x9d mechanism, first disclosed in the patent U.S. Pat. No. 3,902,084. The driven component is moved by mechanical steps in a clamp-extend-unclamp fashion, see e.g. in U.S. Pat. No. 5,751,090.
In the patent U.S. Pat. No. 6,184,609, a piezoelectric motor based on a non-dynamic driving mechanism is disclosed. The mechanism is an alternative to the xe2x80x9cinch-wormxe2x80x9d mechanism and could be denominated a xe2x80x9cmechanical walking mechanismxe2x80x9d. The motor is made of an electromechanical material as a monolithic multilayer unit with at least two independent sets of drive elements that can move two-dimensionally. The motion of each set is characterized by the four sequences of gripping, moving, releasing and returning.
In U.S. Pat. No. 6,184,609, sinusoidal voltage signals are used to excite the drive elements, which results in an elliptical motion of the contact portions of the drive elements. The grip transfer between the different sets of drive elements occurs at essential zero velocity in the drive direction, which means that the entire object to be moved will be accelerated and decelerated within each step. Furthermore, the contact portions of the drive elements may be exposed to wear.
A general problem has been to find waveforms reducing noise and wear associated with velocity variations in the main displacement direction. Also vibrations in the normal direction of the moving object are much related to acoustic sound generation. Analysis and improvements of these properties have been disclosed in U.S. Pat. No. 6,337,532.
Due to different resonance phenomena in the stator and/or moving object, quasi-static motion is limited below certain frequencies. A typical design criterion for quasi-static motors according to prior art is to keep the frequency at least one order of magnitude below fr, where fr is the lowest resonance frequency in the system. Increasing the frequency further will typically induce position accuracy problems, although the resonance frequency is not reached. The absolute velocity of the moving object is thus severely limited.
When stopping the electromechanical motor at a certain arbitrary position, the elements are in general exposed to a certain applied voltage, which now is constant. When maintaining these constant conditions, creeping phenomena may occur, which in turn may change the actual position of the moving object somewhat.
As a summary, general problems with prior-art electromechanical motors are e.g. accuracy problems, noise problems, wear problems, velocity problems and to a certain extent also efficiency problems.
An object of the present invention is to provide improved methods, control devices and motors that reduce noise and wear. Another object of the present invention is to provide improved methods, control devices and motors with increased positioning accuracy, both dynamically and statically. Yet another object of the present invention is to provide motors that are possible to operate in a quasi-static manner at higher frequencies. Further objects of the present invention are also to improve power efficiency and to lower the production costs.
The above objects are achieved by methods, devices and motors according to the enclosed patent claims. In general words, the motors are driven such that the contact portions of the driving elements are moved along smooth trajectories. The velocity along the trajectories are varied in such a way that the average velocity is lower during the time when the element is in contact with the moving object than during the time when the element is free from contact. Preferably, the velocity component in the main displacement direction is non-negligible at the occasion when one set of elements grips the moving object and another releases the moving object. Voltage signals achieving such motions can preferably be selected as sinusoidal functions having an argument that is non-linear in time. It is further preferred to keep the main displacement velocity substantially constant during the time the element is in mechanical contact with the moving object.
When stopping the motor according to the present invention, the actuating sets of elements can be brought into a voltage-free condition, one set at a time, without change the position of the moving object in the main displacement direction.
The contact portions of the elements and the moving object are lapped with such an accuracy, that the normal force applied between the moving object and the stator is large enough to cause elastic deformations of the elements that are in the same order of magnitude or larger than the lapping accuracy. This allows for driving the motor at frequencies very close to the resonance frequencies of the motor, while maintaining the quasi-static motion.
Preferably, the contact portion of the drive elements is narrower in the main displacement direction than the drive element itself. Alternatively, the drive elements are formed by serially arranged bimorphs, to which voltages are applied in opposite manners, always giving a s-shaped stroke. The contact portions are advantageously provided with teeth structures.